This application is related to xe2x80x9cProcess for Conversion of Natural Gas and Associated Light Hydrocarbons to Salable Productsxe2x80x9d by Dennis J. O""Rear, Charles L. Kibby and Russell R. Krug, filed concurrently with this application.
This invention relates to the molecular averaging of various feedstocks to form lube oils.
There is a need for lubricating oils in the C30+ range which have a high viscosity index (VI) and good oxidation stability. The majority of lubricating oils used in the world today are derived from crude oil, and include a petroleum base oil and an additive package. The base oils are refined from crude oil through a plurality of processes such as distillation, hydrocracking, hydroprocessing, catalytic dewaxing, and the like. Hydrocarbons in the lube oil boiling range from these processes needs to be further processed to create the finished base oil. In creating the base oil, the refiner desires to obtain the highest possible yield while preserving the VI of the oil.
Crude oil fractions in the C30+ range often tend to include waxes. Since the presence of wax in lube oil adversely affects various physical properties, such as the pore point and cloud point, the waxy components are typically removed. The waxy components of the oil can be removed using various processes, including solvent dewaxing and/or catalytic dewaxing, both of which tend to provide lower yields at a given VI. It would be highly desirable to have a process that optimizes the yield of lube oil at a given VI.
The use of crude oil as a feedstock for preparing lube oils is limited by the product loss associated with the steps required to remove the waxy components. Further, crude oil is in limited supply, it includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment.
Lube oils can also be prepared from natural gas. This involves converting natural gas, which is mostly methane, to synthesis gas (syngas), which is a mixture of carbon monoxide and hydrogen, and subjecting the syngas to Fischer-Tropsch reaction conditions. An advantage of using fuels prepared from syngas is that they do not contain significant amounts of nitrogen or sulfur and generally do not contain aromatic compounds. Accordingly, they have minimal health and environmental impact.
A limitation associated with Fischer-Tropsch chemistry is that it tends to produce a broad spectrum of products, ranging from methane to wax. While the product stream includes a fraction useful as lube oils, it is not the major product. Product slates for syngas conversion over Fischer-Tropsch catalysts (for example, Fe, Co and Ru) are controlled by polymerization kinetics with fairly constant chain growth probabilities that fix the possible product distributions. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. Methane is produced with high selectivity when chain growth probabilities are low.
It is generally possible to isolate various fractions from a Fischer-Tropsch reaction, for example, by distillation. The fractions include, among others, a gasoline fraction (B.P. about 68-450xc2x0 F./20-232xc2x0 C.), a middle distillate fraction (B.P. about 250-750xc2x0 F./121-399xc2x0 C.), a wax fraction (B.P. about 650-1200xc2x0 F./343-649xc2x0 C.) primarily containing C20 to C50 normal paraffins with a small amount of branched paraffins and a heavy fraction (B.P. above about 1200xc2x0 F./649xc2x0 C.) and tail gases. A suitable fraction for use in preparing a lube oil can be isolated from the product stream by distillation. However, depending on market considerations, it might be advantageous to provide a process that would convert the other fractions into fractions suitable for use in preparing lube oils. The present invention provides such a process.
In its broadest aspect, the present invention is directed to an integrated process for producing hydrocarbons in the lube base oil range, lube base oils and lube oils. As used herein, lube base oils are generally combined with an additive package to provide finished lube oils. Hydrocarbons in the lube base oil range are prepared via molecular averaging of a relatively low molecular weight fraction and a relatively high molecular weight fraction.
The resulting hydrocarbons tend to be waxy unless they are isomerized prior to the molecular averaging step. Isomerization of the hydrocarbons provides a lube base oil, which, when combined with the additive package, provides a lube oil composition. Catalytic isomerization improves the pour point and viscosity index. Hydrotreatment can optionally be performed on the hydrocarbons or lube base oil to hydrotreatment to remove olefins, oxygenates and other impurities.
Depending on the desired physical and chemical properties of the lube oil composition, the product of the molecular averaging reaction can include virtually any combination of hydrocarbons between C20 and C50. Preferably, the lube oil composition includes mostly hydrocarbons in the range of around C30. When preparing a lube base oil composition in the C20 to C50 range, one can combine hydrocarbon materials below C20 and above C50 and subject them to molecular averaging to arrive at a composition in the desired range. When preparing a lube base oil composition in the C30 range, for example, C20 and C40 fractions can be combined and subjected to molecular averaging.
In one embodiment, the process involves performing Fischer-Tropsch synthesis on syngas to provide a range of products, isolating various fractions via fractional distillation, and performing molecular averaging on a relatively low molecular weight fraction and a relatively high molecular weight fraction to provide a product with a molecular weight between the low and high molecular weights, which is suitable for use in preparing a lube base oil composition. In another embodiment, relatively low molecular weight and/or relatively high molecular weight fractions are obtained from another source, for example, via distillation of crude oil, provided that the fractions do not include appreciable amounts (i.e., amounts which would adversely affect the catalyst used for molecular averaging) of thiols, amines, or cycloparaffins.
It may be advantageous to take representative samples of each fraction and subject them to molecular averaging reactions, adjusting the relative proportions of the fractions until a product with desired properties is obtained. Then, the reaction can be scaled up using the relative ratios of each of the fractions that resulted in the desired product. Using this method, one can xe2x80x9cdial inxe2x80x9d a molecular weight distribution which can be roughly standardized between batches and result in a reasonably consistent product.